Process for the selective hydroisomerization of long linear and/or slightly branched paraffins using a catalyst based on a molecular sieve

ABSTRACT

PCT No. PCT/FR96/01364 Sec. 371 Date May 5, 1997 Sec. 102(e) Date May 5, 1997 PCT Filed Sep. 5, 1996 PCT Pub. No. WO97/09397 PCT Pub. Date Mar. 13, 1997The invention concerns a process for the selective hydroisomerisation of compounds containing at least one n-alkane chain containing more than 10 carbon atoms, in which the compound to be treated is brought into contact with a catalyst comprising at least one hydro-dehydrogenating element and at least one molecular sieve with a mono- or bidimensional pore network in which the openings of the accessible pores are delimited by 10 oxygen atoms, and the distance termed the bridge width between the pores is less than 0.70 nm, and in which the catalyst, when subjected to a standard n-heptadecane isomerization test, has a selectivity of at least 70% towards isomerized products for a conversion of 95%. The sieve is preferably a NU-10, NU-23, NU-87, EU-13 or Theta-1 zeolite.

BACKGROUND OF THE INVENTION

The present invention concerns a process for selectivehydroisomerisation of long (more than 10 carbon atoms), linear and/orslightly branched paraffins, in particular for high yield conversion offeeds with high pour points to at least one cut with a low pour pointand a high viscosity index.

High quality lubricants are of fundamental importance for the efficientoperation of modern machines, cars and trucks. However, the quantity ofparaffins which directly originate from untreated crude and which havethe properties to constitute good lubricants is very low when comparedwith the increasing demand in this sector.

Heavy oil fractions containing large amounts of linear or slightlybranched paraffins must be treated to obtain good quality lubricantstock in the best possible yields. A dewaxing operation is used which isintended to eliminate the linear or very slightly branched paraffinsfrom feeds which are then used as lubricant stock or as kerosine or jetfuel.

The linear or very slightly branched high molecular weight paraffinswhich are present in the oils, kerosine or jet fuel produce high pourpoints and thus lead to coagulation at low temperatures. To reduce thepour points, these linear or very slightly branched paraffins must becompletely or partially eliminated.

The dewaxing operation can be effected by extraction using solvents suchas propane or methyl ethyl ketone, using propane or methyl ethyl ketone(MEK) dewaxing. However, such techniques are costly, long and not alwayseasy to carry out.

Catalytic dewaxing, as opposed to solvent dewaxing, is more economicaland can produce products with the desired physico-chemical properties.This is achieved by selective cracking of the longest linear paraffinchains which leads to the formation of lower molecular weight compounds,a portion of which can be eliminated by distillation.

Because of their form-selectivity, zeolites are among the most widelyused dewaxing catalysts. The idea that anticipated their use is thatzeolite structures exist in which the pore openings are such that theyallow long linear or very slightly branched paraffins to enter theirmicroporosity but branched paraffins, naphthenes and aromatics areexcluded. This phenomenon thus leads to selective cracking of linear orvery slightly branched paraffins.

Zeolite based catalysts with intermediate pores such as ZSM-5, ZSM-11,ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-38 have been described for use incatalytic dewaxing by cracking.

Processes using those zeolites can achieve dewaxing by cracking of feedscontaining quantities of less than 50 weight % of linear or veryslightly branched paraffins. However, with feeds containing higherquantities of these compounds, cracking leads to the formation of largequantities of lower molecular weight compounds such as butane, propane,ethane and methane, which considerably reduces the yield of the desiredproducts.

In order to overcome these problems, we have concentrated our researchon the development of catalysts (preferably non ZSM) which could bringabout isomerisation of these compounds.

A number of patents exist in this field, for example Internationalpatent application WO 92/01657 which describes and claims a process fordewaxing feeds with isomerisation in the presence of a group VIII metalat a hydrogen pressure in the range 100 KPa to 21000 KPa and using acatalyst with a pore opening in the range 0.48 nm to 0.71 nm and inwhich the crystallite size is less than 0.5 μm. The catalyst leads toimproved performances as regards yield with respect to prior artcatalysts.

SUMMARY OF THE INVENTION

The invention provides a process for the selective hydroisomerisation ofcompounds containing at least one n-alkane chain containing more than 10carbon atoms, in which the compound to be treated is brought intocontact with a catalyst comprising at least one hydro-dehydrogenatingelement and at least one molecular sieve with a mono- or bidimensionalpore network in which the openings of the accessible pores are delimitedby 10 oxygen atoms, and the distance termed the bridge width between thepores is less than 0.70 nm, and in which the catalyst, when subjected toa standard n-heptadecane isomerisation test, has a selectivity of atleast 70% towards isomerised products for a conversion of 95%.

Advantageously, the process can convert a feed with a high pour point toa mixture with a lower pour point and a high viscosity index.

The feed also comprises linear and/or slightly branched paraffinscontaining at least 10 carbon atoms, preferably 15 to 50 carbon atomsand advantageously 15 to 40 carbon atoms.

The process comprises the use of a catalyst comprising at least onemolecular sieve having at least one pore type which has an openingdelimited by 10 oxygen atoms and which are the largest type of pore inthe structure which is accessible from the exterior. The pore network ofthe zeolite is mono- or bidimensional, preferably monodimensional.

The bridge width between two pore openings (of 10 oxygen atoms), asdefined above, is less than 0.70 nm (1 nm=10⁻⁹ m), preferably in therange 0.50 nm to 0.68 nm, more preferably in the range 0.52 nm to 0.65nm. The crystallite size is preferably less than 2 μm (1 μm=10⁻⁵ m),advantageously less than 1 μm and preferably 0.4 μm.

The catalyst is also characterized in that, for a conversion of theorder of 95% by weight of n-heptadecane (n-C17), it results in aselectivity towards isomerised products of 70% or more, preferably atleast 80%, under a standard n-C17 isomerisation test (SIT) which will bedescribed below. The isomerised products generally contain between about65% and 80% by weight of monobranched products and between about 20% and35% by weight of multibranched products, essentially dibranchedproducts. The term "monobranched products" denotes linear paraffinscontaining a single methyl group, and the term "dibranched products"denotes linear paraffins containing 2 methyl groups which are notcarried by the same carbon atom. Multibranched products can also bedefined by extension. The hydroisomerisation is thus selective.

The catalyst also comprises at least one hydro-dehydrogenating function,for example a group VIII metal and/or a group VIB metal and/or rheniumand/or niobium, and the reaction is carried out under the conditionsdescribed below.

We have surprisingly discovered that one of the determining factors inobtaining high selectivities towards isomerised products is the use ofmolecular sieves which are characterized in that

the opening of the largest pores is delimited by 10 oxygen atoms;

the bridge width must be less than 0.70 nm, preferably in the range 0.50nm to 0.68 nm, more preferably in the range 0.52 nm to 0.65 nm.

This last point in particular is in contradiction to that claimed in theprior art patents cited above such as International application WO92/01657 which has an essential feature a range of pore sizes to producegood yields of isomerised products.

The bridge width is measured using a molecular modelling and drawingmethod such as Hyperchem or Biosym which can construct the surface ofthe molecular sieves concerned and measure the bridge width using theionic radii of the elements present in the sieve framework.

The use of the molecular sieves of the invention under the conditionsdescribed above can produce products with a low pour point and a highviscosity index in good yields.

DETAILED DESCRIPTION OF THE INVENTION

Molecular sieves of the invention which can be used for theisomerisation of linear or slightly branched paraffin hydrocarbons arezeolites, crystallised aluminosilicates such as Theta-1, NU-10, NU-23,EU-13, where the Si/Al ratio is best suited for the desired application.Also among zeolites of the invention is NU-87 zeolite which definitelyhas pores delimited by 10 and 12 oxygen atoms but where accessibility tothe latter is via the pore openings with 10 oxygen atoms. Derivatives ofthe zeolites described above comprising at least one heteroatom in thezeolitic framework such as B, Fe, Ga or Zn are also included in thescope of the invention.

NU-10 zeolite used in the process of the invention and its synthesismethod are described in European patent EP-A-0 077 624. That NU-10zeolite is characterized by the following X ray diffraction table:

    ______________________________________                                        X ray diffraction table for NU-10 zeolite                                     d(A)                  I/I.sub.0                                               ______________________________________                                        10.95 ± 0.25       m to S                                                  8.80 ± 0.14        w to m                                                  6.99 ± 0.14        w to m                                                  5.41 ± 0.10        w                                                       4.57 ± 0.09        w                                                       4.38 ± 0.08        VS                                                      3.69 ± 0.07        VS                                                      3.63 ± 0.07        VS                                                      3.48 ± 0.06        m to S                                                  3.36 ± 0.06        w                                                       3.31 ± 0.05        w                                                       2.78 ± 0.05        w                                                       2.53 ± 0.04        m                                                       2.44 ± 0.04        w                                                       2.37 ± 0.03        w                                                       1.88 ± 0.02        w                                                       ______________________________________                                    

w=weak (l/l₀ between 0 and 20); m=medium (l/l₀ between 20 and 40);S=strong (l/l₀ between 40 and 60); VS=very strong (l/l₀ between 60 and100).

NU-10 zeolite has a Si/Al atomic ratio in the range 8 to 1000.

It has been observed that the catalysts of the invention can becharacterized in a catalytic test known as a standard isomerisation test(SIT) using pure n-heptadecane which is carried out at a partialpressure of 150 kPa of hydrogen and at a partial n-C17 pressure of 0.5kPa, i.e., a total pressure of 150.5 kPa in a fixed bed and at aconstant n-C17 flow rate of 15.4 ml/h using a catalyst mass of 0.5 g.The reaction is carried out in downflow mode. The degree of conversionis regulated by the temperature at which the reaction is carried out.The catalyst subjected to this test is constituted by pure pelletizedzeolite and 0.5% by weight of platinum.

The sieve generally contains at least one hydro-dehydrogenating element,for example at least one group VIII metal, preferably at least one metalselected from the group formed by Pt or Pd, which is introduced into themolecular sieve by dry impregnation or ion exchange, for example, orusing any other method which is known to the skilled person.

The concentration of hydro-dehydrogenating metal(s) introduced,expressed as the % by weight with respect to the mass of molecular sieveused, is generally less than 5% (0.01-5%), preferably less than 1%(0.01-1%) and generally of the order of 0.5% by weight. Under theseconditions, a molecular sieve of the invention must produce, for adegree of conversion of n-C17 of the order of 95% by weight (the degreeof conversion is regulated by the temperature) a selectivity towardsisomerised products of 70% by weight or more, preferably at least 80% byweight.

The isomerisation selectivity in the standard n-C17 isomerisation test(SIT_(n-C17)) is defined as follows: ##EQU1## leading to a n-C17conversion of the order of 95%.

C₁₇₋ products are compounds containing less than 17 carbon atomsregardless of their degree of branching.

When treating an actual feed, the molecular sieve of the invention isfirst formed. In a first variation, the molecular sieve can have atleast one group VIII metal deposited on it, preferably selected from thegroup formed by platinum and palladium, and can then be formed using anytechnique which is known to the skilled person. In particular, it can bemixed with a matrix which is generally amorphous, for example a wetalumina gel powder. The mixture is then formed, for example by extrusionthrough a die. The amount of molecular sieve in the mixture obtained isgenerally in the range 0.5% to 99.9%, advantageously in the range 10% to90% by weight with respect to the mixture (molecular sieve+matrix),preferably in the range 20% to 70%.

In the following text, the term "support" is used for the mixture of themolecular sieve+matrix.

Forming can be carried out with matrices other than alumina, such asmagnesia, amorphous silica-aluminas, natural clays (kaolin, bentonite,sepiolite, attapulgite) and using other techniques such as pelletizingor bowl granulation.

The hydrogenating group VIII metal, preferably Pt and/or Pd, can also bedeposited on the support using any process which is known to the skilledperson for depositing a metal on a molecular sieve. A competing cationexchange technique can be used where the competitor is preferablyammonium nitrate, the competition ratio being at least about 20 andadvantageously about 30 to 200. In the case of platinum or palladium, aplatinum tetramine complex or a palladium tetramine complex is normallyused: these latter are thus practically completely deposited on themolecular sieve. This cation exchange technique can also be used todeposit the metal directly onto molecular sieve powder before any mixingwith a matrix.

Deposition of the group VIII metal (or metals) is generally followed bycalcining in air or oxygen, usually between 300° C. and 600° C. for 0.5to 10 hours, preferably between 350° C. and 550° C. for 1 to 4 hours.Reduction in hydrogen can then follow, generally at a temperature in therange 300° C. to 600° C. for 1 to 10 hours; preferably, in the range350° C. to 550° C. for 2 to 5 hours.

The platinum and/or palladium can also be deposited not directly on themolecular sieve but on the alumina binder before or after the formingstep, using anion exchange with hexachloroplatinic acid,hexachloropalladic acid and/or palladium chloride in the presence of acompeting agent, for example hydrochloric acid. In general, afterdepositing the platinum and/or palladium, the catalyst is, as before,calcined as before then reduced in hydrogen as indicated above.

Advantageously, the feeds which can be treated using the process of theinvention are fractions with relatively high pour points which latter itis desired to reduce.

The process of the invention can be used to treat various feeds fromrelatively light fractions such as kerosines and jet fuels up to feedswith higher boiling points such as middle distillates, vacuum residues,gas oils, middle distillates from FCC (LCO and HCO) and hydrocrackingresidues.

The feed to be treated is usually a C10+ cut with an initial boilingpoint of more than about 175° C. or a C20+ cut with an initial boilingpoint of more than 315° C., preferably a heavy cut with an initialboiling point of at least 380° C. The process of the invention isparticularly suitable for the treatment of paraffinic distillates suchas middle distillates which include gas oils, kerosines, jet fuel andall other fractions where the pour point and viscosity are to be adaptedto bring them within specifications.

Feeds which can be treated using the process of the invention cancontain paraffins, olefins, naphthenes, aromatics and also heterocycles,along with a large proportion of high molecular weight n-paraffins andvery slightly branched high molecular weight paraffins,

The reaction can be carried out such that the degree of crackingreactions remains sufficiently low to render the process economicallyviable. The number of cracking reactions is generally less than 20% byweight.

Typical feeds which can advantageously be treated in accordance with theinvention generally have a pour point above 0° C., more usually above15° C. The products resulting from treatment in accordance with theinvention have pour points of less than 0° C., preferably less thanabout -10° C.

These feeds contain more than 30% and up to about 90%, and in some casesmore than 90% by weight of high molecular weight n-paraffins (n-alkanes)containing more than 10 carbon atoms, and of paraffins containing morethan 10 carbon atoms which are very slightly branched and also of highmolar weight. The process is of particular importance when thisproportion is at least 60% by weight.

Non limiting examples of other feeds which can be treated in accordancewith the invention are bases for lubricating oils, synthesised paraffinsfrom the Fischer-Tropsch process, polyalphaolefins with high pourpoints, synthesised oils, etc. The process can also be applied to othercompounds containing an n-alkane chain as defined above, for examplen-alkylcycloalkane compounds, or containing at least one aromatic group.

The operating conditions under which the hydroisomerisation of theinvention is carried out are as follows:

the reaction temperature is in the range 170° C. to 500° C., preferablyin the range 180° C. to 450° C., advantageously 180-400° C.;

the pressure is in the range 1 bar to 250 bar, preferably in the range10 bar to 200 bar;

the hourly space velocity (vvh, the volume of feed injected per unitvolume of catalyst per hour) is in the range about 0.05 h⁻¹ to about 100h⁻¹ , preferably in the range about 0.1 h⁻¹ to about 30 h⁻¹.

The feed and the catalyst are brought into contact in the presence ofhydrogen. The quantity of hydrogen used, expressed in liters of hydrogenper liter of feed, is in the range 50 liters to about 2000 liters ofhydrogen per liter of feed, preferably in the range 100 liters to 1500liters of hydrogen per liter of feed.

The feed to be treated preferably has a nitrogen compound concentrationof less than about 200 ppm by weight, preferably less than 100 ppm byweight. The sulphur concentration is less than 1000 ppm by weight,preferably less than 500 ppm, and more preferably less than 200 ppm byweight. The concentration of metals such as Ni or V in the feed isextremely low, i.e., less than 50 ppm by weight, preferably less than 10ppm by weight and more preferably less than 2 ppm by weight.

The compounds obtained by the process of the invention are essentiallymonobranched, dibranched and multibranched with methyl groups. As anexample, in the case of a feed constituted by pure n-heptadecane(n-C17), methylhexadecane compounds are selectively obtained, mainly2-methylhexadecane, also the dibranched compounds 2,7-; 2,8-; 2,9-;2,10-; and 2,11-dimethylpentadecane. The total of the isomerisedproducts represents more than 70% by weight of the products obtained,with 95% conversion. The isomerised carbon atoms are separated by adistance of at least the bridge width.

The following examples illustrate the invention without in any waylimiting its scope. They are given for a feed constituted byn-heptadecane (standard isomerisation test, SIT), or for a hydrocrackingresidue.

EXAMPLES Example 1

Catalyst C1, in Accordance with the Invention

The starting material was a NU-10 zeolite in its H form with a globalSi/Al ratio of about 30, a pore opening delimited by 10 oxygen atoms anda bridge width, i.e., the distance between two pore openings, of 0.55nm. The crystallites of the NU-10 zeolite were in the form of needlesless than 1 μm in length and a width in the range 0. 1 μm to 0.4 μm.

The NU-10 zeolite was dry impregnated with a solution of [Pt(NH₃)₄ ]Cl₂to obtain, after calcining and reduction at 450° C., a Pt content of0.5% by weight of platinum on the zeolite.

0.5 g of this platinum-charged zeolite, which had been pelletized (200μm to 300 μm granulometric fraction) was introduced into a fixed bedreactor.

The standard n-heptadecane isomerisation test (SIT) was then carried outat a partial pressure of 150 kPa of hydrogen and a partial pressure of0.5 kPa of n-C17, i.e., a total pressure of 150.5 kPa, on a fixed bedwith a constant n-C17 downflow rate of 15.4 ml/h and a catalyst mass of0.5 g. The degree of conversion was regulated by the temperature atwhich the reaction was carried out.

In this example, the temperature required to achieve 95% by weightconversion of n-C17 was 190° C. At this temperature, the selectivity forisomerised products was 93% by weight. The selectivity is defined asfollows: ##EQU2##

The selectivity towards monobranched compounds was 67.4% and formultibranched compounds it was 25.6%.

Example 2

Catalyst C2, Not in Accordance with the Invention

The starting material was a USY zeolite in its H form with a globalSi/Al ratio of about 5, a pore opening delimited by 12 oxygen atoms anda bridge width, i.e., the distance between two pore openings, of morethan 0.7 nm.

The USY-H zeolite was dry impregnated with a solution of [Pt(NH₃)₄ ]Cl₂to obtain, after calcining and reduction at 450° C., a Pt content of0.5% by weight of platinum on the zeolite.

0.5 g of this platinum-charged zeolite, which had been pelletized (200μm to 300 μm granulometric fraction) was introduced into a fixed bedreactor.

The standard n-heptadecane isomerisation test (SIT) was then carried outat a partial pressure of 150 kPa of hydrogen and a partial pressure of0.5 kPa of n-C17, i.e., a total pressure of 150.5 kPa, on a fixed bedwith a constant n-C17 downflow rate of 15.4 ml/h and a catalyst mass of0.5 g. The degree of conversion was regulated by the temperature atwhich the reaction was carried out.

In this example, the temperature required to achieve 95% by weightconversion of n-C17 was 220° C. At this temperature, the selectivity forisomerised products was only 16% by weight. The selectivity is definedas in Example 1.

The standard isomerisation test is thus a means of selecting catalysts.

Example 3

In Accordance with the Invention

The zeolite used in this example was the same NU-10 zeolite as that usedin Example 1.

The zeolite was milled with SB3 type alumina provided by Condea. Themilled paste was extruded through a 1.2 mm diameter die. The extrudateswere calcined at 500° C. for 2 hours in air then dry impregnated with atetramine platinum chloride solution [Pt(NH₃)₄ ]Cl₂, then calcined inair at 550° C. The platinum content in the final catalyst C3 was 0.7% byweight and the zeolite content, with respect to the whole catalyst mass,was 60% by weight.

Evaluation of Catalyst C3 in Hydroisomerisation of a HydrocrackingResidue from a Vacuum Distillate.

The characteristics of the feed were as follows:

    ______________________________________                                        Sulphur content (ppm by weight)                                                                    12                                                       Nitrogen content (ppm by weight)                                                                   2                                                        Pour point (° C.)                                                                           +30                                                      Initial boiling point                                                                              104                                                       5%                  325                                                      10%                  385                                                      50%                  452                                                      90%                  520                                                      95%                  536                                                      End point            573                                                      ______________________________________                                    

The prepared catalyst was then used to prepare a lubricant stock byhydroisomerisation of the above feed.

The catalyst was first reduced in hydrogen at 450° C. before thecatalytic test, in situ in the reactor. Reduction was carried out instages. It consisted of a 2 hour stage at 150° C., then raising thetemperature to 450° C. at 1° C./min, then a 2 hour stage at 450° C.During this reduction procedure, the hydrogen flow rate was 1000 litersof H₂ per liter of catalyst.

The reaction was carried out at 300° C. at a total pressure of 12 MPa,an hourly space velocity of 0.9 h⁻¹ and a hydrogen flow rate of 1000liters of H₂ per liter of feed. Under these operating conditions, thegross conversion of 400- was 65% and the lubricant stock yield was 86%.

The characteristics of the oil after hydroisomerisation are shown in thetable below.

    ______________________________________                                        Viscosity index VI  136                                                       Pour point          -18                                                       Oil/feed yield (% by weight)                                                                      86                                                        ______________________________________                                    

This example shows the importance of using a catalyst of the inventionwhich can reduce the pour point of the initial feed for a hydrocrackingresidue, while retaining a high viscosity index (VI).

The present invention has been illustrated with the aim of producing anoil, but other aims can be achieved. In general, the invention can beused to produce multiple, localized branching.

We claim:
 1. A process for the selective hydroisomerisation of compoundscontaining at least one n-alkane chain containing more than 10 carbonatoms, in which said compound to be treated is brought into contact witha catalyst comprising at least one hydro-dehydrogenating element and atleast one molecular sieve with a mono- or bidimensional pore network inwhich the openings of the accessible pores are delimited by 10 oxygenatoms, and the distance termed the bridge width between the pores isless than 0.70 nm, the molecular sieve is NU-23, NU-87 or EU-13 and inwhich said catalyst, when subjected to a standard n-heptadecaneisomerisation test, has a selectivity of at least 70% towards isomerisedproducts for a conversion of 95%.
 2. A process according to claim 1, inwhich the bridge width is in the range 0.50 nm to 0.68 nm.
 3. A processaccording to claim 1, in which the bridge width is in the range 0.52 nmto 0.65 nm.
 4. A process according to claim 1, in which the molecularsieve has a crystallite size of less than 2 μm.
 5. A process accordingto claim 1, in which the molecular sieve has a crystallite size of lessthan 1 μm.
 6. A process according to claim 1, in which the molecularsieve has a crystallite size of less than 0.4 μm.
 7. A process accordingto claim 1, in which the molecular sieve is NU-23.
 8. A processaccording to claim 1, in which the molecular sieve is NU-87.
 9. Aprocess according to claim 1, in which the molecular sieve is EU-13. 10.A process according to claim 1, in which the hydro-dehydrogenatingelement is selected from the group consisting of group VIII metals,group VIB metals, rhenium and niobium.
 11. A process according to claim1, in which the catalyst contains a matrix, and 0.5-99.9% by weight ofmolecular sieve, with respect to the mixture of matrix and sieve, andless than 5% by weight of hydro-dehydrogenating metal with respect tothe sieve.
 12. A process according to claim 11, in which the catalystcontains 10%-90% by weight of sieve with respect to the mixture ofmatrix and sieve.
 13. A process according to claim 1, in which thepressure is in the range 1 bar to 250 bars, the temperature is in therange 170° C. to 500° C., the hourly space velocity is in the range 0.05h⁻¹ to 100 h⁻¹, and the hydrogen concentration is in the range 50 litersto 2000 liters of hydrogen/liters of feed.
 14. A process according toclaim 13, in which the temperature is in the range 180-450° C.
 15. Aprocess according to claim 14, in which the pressure is in the range 10bars to 200 bars.
 16. A process according to claim 15, in which thehydrogen concentration is in the range 100 liters to 1500 liters ofhydrogen/liter of feed.
 17. A process according to claim 1, in which thecompound to be treated in selected from the group consisting ofn-alkanes, n-alkylcycloalkanes and compounds containing at least onearomatic group.
 18. A process according to claim 1, in which thecompound to be treated is present in a feed with an initial boilingpoint of more than 175° C.
 19. A process according to claim 1, in whichthe compound to be treated is present in a feed with an initial boilingpoint of at least 380° C.
 20. A process according to claim 1, in whichthe compound to be treated comprises an n-alkane chain containing 15 to50 carbon atoms.
 21. A process according to claim 1, in which thecompound to be treated comprises an n-alkane chain containing 15 to 40carbon atoms.
 22. A process according to claim 1, in which the compoundto be treated is present in a hydrocarbon feed selected from the groupformed by middle distillates, vacuum residues, hydrocracking residues,paraffins from the Fischer-Tropsch process, synthesised oils, gas oilcuts, middle distillates from FCC, lubricant stocks, andpolyalphaolefins.
 23. A process for the selective hydroisomerisation ofcompounds containing at least one n-alkane chain containing more than 10carbon atoms, in which said compound to be treated is brought intocontact with a catalyst comprising at least one hydro-dehydrogenatingelement and at least one molecular sieve with a mono- or bidimensionalpore network in which the openings of the accessible pores are delimitedby 10 oxygen atoms, and the distance termed the bridge width between thepores is less than 0.70 nm, the molecular sieve is Nu-23, Nu-87 orEu-13, and contains boron, gallium or zinc, and in which said catalyst,when subjected to a standard n-heptadecane isomerisation test, has aselectivity of at least 70% towards isomerised products for a conversionof 95%.
 24. A process for the selective hydroisomerisation of compoundscontaining at least one n-alkane chain containing more than 10 carbonatoms, in which said compound to be treated is brought into contact witha catalyst comprising at least one hydro-dehydrogenating element and atleast one molecular sieve with a mono- or bidimensional pore network inwhich the openings of the accessible pores are delimited by 10 oxygenatoms, and the distance termed the bridge width between the pores isless than 0.70 nm, the molecular sieve is Eu-13, and in which saidcatalyst, when subjected to a standard n-heptadecane isomerisation test,has a selectivity of at least 70% towards isomerised products for aconversion of 95%.
 25. A process according to claim 24, wherein themolecular sieve contains B, Ga or Zn.
 26. A process for the selectivehydroisomerisation of compounds containing at least one n-alkane chaincontaining more than 10 carbon atoms, in which said compound to betreated is brought into contact with a catalyst comprising at least onehydro-dehydrogenating element and at least one molecular sieve with amono- or bidimensional pore network in which the openings of theaccessible pores are delimited by 10 oxygen atoms, and the distancetermed the bridge width between the pores is less than 0.70 nm, themolecular sieve is Nu-10 containing boron, and in which said catalyst,when subjected to a standard n-heptadecane isomerisation test, has aselectivity of at least 70% towards isomerised products for a conversionof 95%.